Because the airlines have encouraged "hand luggage only" flights (both BA and EZ) but there isn't enough overhead space for everybody to take a roll-on. That means there is a stampeded for the aeroplane (or more and more speedy boarders) to try and get what space is available. I'm now finding that when Mrs G and I book "up front" some even speedier boarder has put their carry on at the front of the bus and gone to a cheaper seat at the back.